An image forming apparatus is arranged to form an electrostatic latent image on the surface of a photoconductor and giving toner to the electrostatic latent image for development. An example thereof is described below. Firstly, a photoconductor is charged by a charging device and then the charged surface of the photoconductor is partly exposed by an exposure device to form an electrostatic latent image. Then, a developing bias is applied to attract the toner from a developing roller onto the electrostatic latent image on the photoconductor. Accordingly, it is important for ensuring the quality of a printed material to cause a needed amount of toner to exactly or faithfully adhere to the electrostatic latent image in order to output clear and appropriately dark printed images. It is therefore preferable that the toner has an appropriate charge quantity.
In the image forming apparatus, however, the charging characteristics of toner stored in a buffer will change with time because the toner deteriorate due to repeated friction with a restriction blade, a supply roller, and others. At an initial stage, toner has good charging characteristics and little variation in charge quantity. This toner is charged by the restriction blade for development to have an appropriate charge quantity and is attracted onto the developing roller (hereinafter, referred to as “normally-charged toner”). On the other hand, the toner becomes hard to be charged due to some causes, for example, when an external additive comes off due to friction. This generates toner having a low charge quantity (hereinafter, referred to as “low-charged toner”). This low-charged toner may adhere to the background area of the electrostatic latent image to which the toner actually should not adhere. This results in image fogging during printing, leading to degradation in the quality of a printed material. Furthermore, as deterioration of the toner further progresses, some toner particles come to be charged oppositely to the normally-charged toner (hereinafter, referred to as “oppositely-charged toner”). The oppositely-charged toner tends to adhere to the background area rather than to the image area.
When images are continuously printed at high coverage (high printing rate), the toner in the buffer is consumed at once and accordingly a large volume of new toner is added into the buffer at a time. Thus, newly added toner and deteriorated toner are mixed. Due to this mixing of new and old toners, toner charge quantity distribution is broadened. This distribution is broader not only than the case of only new toner but also than the case of only deteriorated toner. In other words, a ratio of normally charged toner decreases, and ratios of the toner having a higher charge quantity than the normally-charged toner, the low-charged toner, and the oppositely-charged toner increase. The oppositely-charged toner may be electrically connected with the normally-charged toner to form combined toner. Such combined toner may also cause image fogging. To facilitate separation of the combined toner, therefore, Patent Literature 1 discloses a developing apparatus arranged to adjust, for a fixed period after toner is replenished, at least one of frequency and amplitude of an alternate current component of a developing bias to be larger than a condition for normal image formation.